The present invention relates to a traffic information providing system for creating traffic information to be used in a car navigation system. In particular, the invention relates to a traffic information providing system suitable for generating traffic information relating to free traffic flow conditions. Moreover, the invention relates to a method for generating traffic information to be used in a car navigation system, a data storage device storing traffic information generated according to the method of the invention and to a car navigation system comprising such a data storage device.
The document US2005/0093720A1 teaches a traffic information providing system for creating traffic information to be used in a car navigation system. The traffic information providing system comprises a unit for getting traffic information data relating to a travel time of links making up a road on a map. The data getting unit moreover acquires map data relating to the link, wherein the map data comprises e.g. information on the road type of the link, on a length of the link and on the coordinates of endpoints of the link. The system according to US2005/0093720A1 teaches to delete abnormal data when there is an abnormality in the acquired traffic information and to apply a statistical processing for calculating a probability distribution of travel times of links making up the entire route and to calculate a probability for a specific travel time range. If no traffic information is provided for a specific link, the travel time associated to such link is estimated either based on a congestion degree and a congestion link of the non-provision link or estimates the travel time associated to the link based on traffic information of links in the vicinity of the non-provision link. The latter estimate uses traffic information relating to links within a predetermined distance of e.g. 2 kilometers.
In a technique disclosed in JP-A-7-129893, the average vehicle speed of a link where no traffic information is provided is calculated based on traffic jam or congestion information included in external information. The complementary information for the non-provision link is calculated based on a calculated average vehicle speed and the travel time for the link is also estimated based on the average vehicle speeds at specific times of the day or at specific days of the week (e.g. holidays, work days, etc.). The technique disclosed in JP-A-7-129893 is intended to be applied a for vehicle route guiding system. However, traffic information is updated from moment to moment, and when traffic information data received are also included in the traffic information database used for calculating the average, a required memory capacity and correspondingly a required processor capacity becomes enormous. In such a vehicle route guiding system, it is difficult to estimate a travel time of a non-provision link on a real-time basis, since the memory capacity and the processing performance are limited. In addition, it is very inefficient to perform the same data processing procedures for individual vehicle route guiding systems over and over again.
The document JP-A-2002-260142 discloses a technique where data about runtimes of a road section including the road in question are collected and stored in a runtime memory table having runtime data base where runtime data are collected. In order to estimate a runtime, the runtime memory table is searched for a pattern similar in runtime and a runtime memory value for the similar pattern found is used as a predicted runtime. By repeating such operations, a plurality of candidates of the predicted runtime are found. In the technique disclosed in JP-A-2002-260142, even when a prediction error within a given probability is found, the error may fail to fall in a practical range. For example, the prediction error within a probability of 90% may become larger than a mean value of the probability distribution of travel times, such that the thus acquired statistical information is not of any practical use.
In general, the estimation of travel times or links where no traffic data is provided is performed based on either road traffic sensor data or on the use of limiting speed data. Since the road traffic sensors are managed by public authorities, it is generally difficult to get such data. The limiting speed data are rarely provided in map data. However, map data and traffic information data often include average travel times relating to a specific link. Conventional traffic information providing systems therefore often use the average travel times provided by the data providers as an estimate for the travel time under free traffic conditions for links where no detailed traffic information is provided. However, the average travel times provided in the map data are often of poor quality, such that the estimates based on such average travel times often suffer from large estimation errors. Since the travel time estimates for free traffic flow are often used as a basis for estimates under congested conditions, the estimation error will be propagated to further processing steps and tends to increase.
In view of the above, it is very important to provide a precise estimate of the travel times under free traffic flow conditions at e.g. midnight as a sound basis for further estimation procedures using the latter travel time estimate or free traffic flow conditions.